This invention relates to a family of compounds which can be crosslinked by photon irradiation and to a process for using these compounds. The photons used for irradiation are photons issuing from either visible or invisible radiation, particularly ultraviolet radiation, X-radiation or gamma radiation.
The phenomenon of crosslinking of so-called negative resins is well known. Resins such as these harden under the effect of photon irradiation by the establishment of "bridging" bonds between the various polymer chains which they contain. These bonds result in the formation of a three-dimensional network. From the point of view of the intrinsic properties of the resins, this is reflected in an increase in their molecular weight which is responsible for their insolubility in developer solvents.
Commercially available crosslinkable resins are formed:
either by a polymer containing both a crosslinkable group and a photosensitive group:
or by a "system" containing both a polymer having a crosslinkable entity and a photoinitiator or photosensitiser compound.
The sensitivity of two known crosslinkable compositions is given in the following by way of order of magnitude:
4 mJ/cm.sup.2 (in photon irradiation at a wavelength of 404.7 nm) for the resin KPR (a Kodak product);
1.3 mJ/cm.sup.2 (in photon irradiation at wavelengths of 351.1 and 363.8 nm) for the epoxy resin commercially available under the name "PSE-2"; a sensitivity of this order requires an exposure time of around 10 seconds under normal working conditions in a factory.
By virtue of the present invention, it is possible to obtain excellent sensitivity levels, i.e. in favourable cases of the order of one tenth of a millijoule per square centimeter.
In addition, the compounds according to the invention have other advantages which will be discussed hereinafter.
According to the invention, there is provided a family of compounds crosslinkable by photon irradiation comprising at least one copolymer of which the chemical formula comprises a thiirane ring: ##STR2## wherein said polymer contains at least one monomer formed by 2,3-epithiopropyl methacrylate corresponding to the developed formula: ##STR3## wherein R is selected from the group consisting of an alkyl group of 1 to 4 carbon atoms and hydrogen.
According to one aspect of the invention, the above monomer is copolymerized with a vinyl monomer corresponding to the formula: ##STR4## in which R' is either a radical H or an alkyl group C.sub.n H.sub.2n +1 where n is an integer from 1 to 10, R" being an alkyl group containing from 1 to 5 carbon atoms.
According to another aspect of the invention, said family additionally comprises at least one of the following two salts:
an aryl diazonium salt corresponding to the general formula: ##STR5## and an aryl iodonium salt corresponding to the formula: ##STR6## where n=0 or 1.
In these formulae:
the substituent .DELTA. denotes a radical or a plurality of radicals such as:
--OH; --NH.sub.2 ; --CHO; --NO.sub.2 ; --OCH.sub.3 ; ##STR7##
the letters a and b denote integers from 1 to 5;
the element M is a metal such as Fe, Sn, Sb or Bi or another element such as B, P, As;
X denotes a halogen atom, such as F or Cl;
T.sub.1 and T.sub.2 denote aromatic radicals (which may be identical) containing from 4 to 20 carbon atoms (phenyl, thienyl, furanyl, pyrazolyl);
Y denotes a radical of the following group: ##STR8##
R.sub.3 --N (with R.sub.3 =H or an alkyl or acyl radical or a C--C bond); ##STR9## (where R.sub.1 and R.sub.2 represent identical or different radicals such as H or a C.sub.1-4 -alkyl radical or a C.sub.2-4 -alkenyl radical).
The copolymer of 2,3-epithiopropyl methacrylate and the vinyl monomer may be obtained as follows:
Glycidyl methacrylate corresponding to the formula: ##STR10## is reacted with thiourea corresponding to the formula: ##STR11## at ambient temperature in a mixture of water and ethanol. The reaction gives 2,3-epithiopropyl methacrylate corresponding to the formula: ##STR12##
This compound is extracted with ether. The solution is dried with sodium sulphate. The ether is removed in vacuo and the remaining product is distilled in vacuo.
The epithiopropyl methacrylate thus obtained is mixed with a quantity of vinyl monomer in a proportion of from 20 to 60% by weight thereof in a solvent, such as benzene. Azobis-isobutyronitrile, which acts as copolymerisation catalyst, is then added to the resulting mixture. The mixture is heated under nitrogen at 80.degree. C. over a period ranging from one to several hours. After cooling, the copolymer is obtained by precipitation with methanol, followed by drying in vacuo.
For example, the vinyl monomer may be butyl methacrylate or ethylacrylate.